2 Answers
2

It can supply orbital data or almanac for the GPS satellites to the
GPS receiver, enabling the GPS receiver to lock to the satellites
more rapidly in some cases.

The network can provide precise time.

The device captures a snapshot of the GPS signal, with approximate time,
for the server to later process into a position.

Accurate, surveyed coordinates for the cell site towers allow better
knowledge of local ionospheric conditions and other conditions
affecting the GPS signal than the GPS receiver alone, enabling more precise
calculation of position. (See also Wide Area Augmentation System and
CellHunter.)

Calculation of position by the server using information from the GPS
receiver

The assistance server has a good satellite signal, and plentiful
computation power, so it can compare
fragmentary signals relayed to it by
GPS receivers, with the satellite
signal it receives directly, and then
inform the GPS receiver or emergency
services of the GPS receiver's
position.

Many mobile phones combine A-GPS and
other location services including
Wi-Fi Positioning System and cell-site
triangulation and sometimes a hybrid
positioning system.[4]

guys, are you illiterate? Did you read the wikipedia article thoroughly and find a connection between Wi-Fi and A-GPS? Assisted GPS has no direct relation to the Wi-Fi. The reason for turning-on your Wi-Fi is to utilize triangulation made by nearby Wi-Fi Hotspots, as Sam said below.
–
ComptrolJan 14 '11 at 8:19

3

@Comptrol: Assisted GPS is an umbrella term to describe any techniques used to get faster or more accurate GPS positions; and Wifi positioning is just one technique used by a device to assist the GPS in getting better "Time to First Fix" and/or Accuracy
–
Lie RyanJan 14 '11 at 9:08

"Assistance falls into two categories" ← None of these described categories need Wi-Fi. Only the last sentence has something to do with Wi-Fi, but it is very vague. Sam's answer should be the accepted answer.
–
Nicolas RaoulOct 21 '13 at 1:20

If I'm not mistaken it is to access Google's (presumably large) wireless MAC address location database which like Skyhook (a competing service, also previously used on iOS devices) allows a handset to scan for wireless networks nearby and send their MAC addresses (possibly SSIDs as well, I haven't looked into it too depply) off to Google to compare to their database and fire back an approximate location based on their data collection (which you consent to help provide by enabling this feature).

To the answerer mentioning A-GPS, I think the OP was already using A-GPS and using Google Maps over a 3G connection but had Wi-Fi disabled.

I believe this technique is achieved similarly with the use of cell towers, measuring the signal strength between the connected and neighbouring towers and comparing it to a stored database (such as Ofcom's attempt available in the UK), Location Area Code (LAC) and Cell ID (CID). For anyone curious you can find these on most Android phones using the *#*#4636#*#* code in your dialer although for whatever reason it's encoded in hex so just convert it back to decimal if you like.

TL;DR - Triangulate your location quicker using Wi-Fi signals while your GPS gets a satellite lock, A-GPS serves the same purpose but achieves this using a different method (Secure User Plane Location I think, if anyone wants to read up on it).